With an estimated seven billion people and counting, the world's population will only get hungrier. The advent of fertilizers and high-yield crops have helped growers keep pace with the demand for food.

But there's an unintended crop flourishing around the world that is not always so beneficial. Microscopic, plantlike organisms called algae thrive on the excess nutrients—like nitrogen and phosphorus—found in fertilizers that make their way from backyards and fields, producing blooms that can sometimes be seen from space.

Combined with warming temperatures and water circulation patterns, coastal areas such as Qingdao, the Gulf of Mexico, and the U.S. West Coast—as well as freshwater systems like the Great Lakes—are no strangers to enormous algae blooms that can turn the water green or red. (Related: "Harmful Algae Blooms Plague Lake Erie Again.")

Some of these blooms can create dead zones, or areas that are deprived of oxygen, in the water. And some algal species can also produce toxins that wreak havoc on human livers and neurological functions and cause seizures in marine mammals. (Related: "Sea Lion Seizures May Result From Toxic Algae.")

"There's no question in my mind that we are seeing a global increase in the frequency and severity of these [blooms]," said David Caron, a researcher at the University of Southern California (USC) in Los Angeles who studies harmful algal blooms.

—Jane J. Lee

Photograph from China Daily/Reuters

Green With Algae

Tan fingers of sediment and green swirls of algae are visible in Lake Erie on March 21, 2012.

According to an April 16 study in the journal Proceedings of the National Academy of Sciences, scientists found that a whopping algae bloom in 2011 was caused by agricultural practices, warming temperatures, weak lake circulation, and heavy precipitation.

At its peak, the bloom covered 1,930 square miles (5,000 square kilometers) of the nearly 10,000-square-mile (25,600-square-kilometer) lake.

The study authors warned that such blooms would become a more frequent occurrence unless steps were taken to manage the lake.

A boat motors through a 2011 algae bloom on Lake Erie near Toledo, Ohio, in an image taken for an article on fertilizing the world in the May 2013 issue of National Geographic magazine.

In addition to causing numerous economic headaches for tourists and fishers, these blooms can also cause health problems.

Not all algae blooms produce toxins, said Caron, USC's harmful algal bloom expert. But for those that do, in freshwater systems the culprit is usually blue-green algae, or cyanobacteria. The common toxins some produce are called microcystins, he said. "They're very harmful to your liver." (Read about sea otters poisoned by microcystins.)

Common toxins produced by algae blooms in the ocean are neurotoxins that can—in high enough quantities—shut down a person's heart, stop their breathing, cause them to lose their short-term memory, or send them into convulsions, he added.

Photograph by Peter Essick, National Geographic

Brush Strokes

Two people row their way across algae-infested Chaohu Lake (map), China, in a 2009 picture.

Although the country has spent billions of dollars trying to clean up its waterways, China's lakes and rivers still host algae blooms that can turn the environs into a watercolor painting.

Researchers are still trying to figure out why some types of algae grow in some circumstances and not in others, said USC's Caron.

"Miracle-Gro has way more nutrients than your lawn can use, and that excess gets washed into the street," he explained. Then those nutrients make their way into the watershed, where they can trigger algae blooms.

People aren't the only ones affected by massive algal blooms, and the consequences for organisms like fish can be fatal. If the blooms are thick enough, algae can clog their gills, reducing the animals' ability to take in oxygen and get rid of waste.

The blooms can also create dead zones: When the massive amounts of algae die, bacteria start to decompose the organic material, and in the process they suck all the oxygen out of the water. This leaves little oxygen behind for fish or other organisms to breathe. (Related:"World's Largest Dead Zone Suffocating Sea.")

Photograph by Tom Archer, NSF

Green Goo

A fisher cups algae-choked water from China's Chaohu Lake in 2009.

There are no easy fixes for algae blooms, said USC's Caron. Many times, the solutions people have come up with to eradicate an algae bloom just introduce new problems.

One technique that some have tried is sprinkling copper sulfate onto the blooms. Copper kills the algae, but the metal can also be harmful to other organisms, he explained. (Learn about copper sulfate.)

Other people have tried spraying fine particles of clay onto a bloom, which clumps up the algae. Once the clumps are heavy enough, they sink to the bottom, Caron said, where they settle on the sediment.

"The difficulty there [is] you're taking a potentially toxic algae that's spread out [on the surface] and concentrating them on the bottom," smothering bottom-dwelling organisms in toxic materials, he explained.

In any case, these are all temporary strategies—Caron isn't currently aware of any truly effective solution to treat algae blooms.

Education "and prevention are probably better strategies."

Photograph from Reuters

Healing

Although blooms, such as the toxic algae bloom in California's Klamath River (above), are becoming more frequent, Caron is hopeful. Water systems are renewable, he said.